Low power two-way wireless communication system for electronic shelf labels

ABSTRACT

A two-way low-power communication system for an electronic shelf label system uses a multiplicity of labels, each of which has an antenna and a diode. Data to be received at the label is Manchester encoded and 100% AM modulated onto a spread-spectrum RF signal, emitted from a broadcast antenna in the store ceiling. Preferably the RF is around 2.4 Ghz, and the spreading is via direct sequence phase shift keying with a chipping rate at least ten times the RF frequency. A 63-bit sequence may be used. The label detects the RF energy, and a simple comparator followed by a manchester decoder extrats the digital data stream. Data to be set from the label is communicated via a selective modulation of the diode with an offset signal, preferably in the range of 1 to 10.7 Mhz. During this time the ceiling broadcast antenna is emitting a spread-spectrum signal that is not data-modulated in any way. Energy re-emitted by the label antenna is picked up by a ceiling-mounted receiving antenna, and the energy is diverted to two sidebands during times when the diode is being modulated. The received signal is spread-spectrum demodulated and band-pass filtered so that only sideband energy is processed. The label&#39;s outbound data stream is recovered by digital signal processing techniques from the sideband energy. The result is a minimization of cost and complexity in the label and an optimal exploitation of the bandwidth given the regulatory framework thereof.

This application which is a division of the application Ser. No.08/398,905 filed on Mar. 6, 1995 and of which the following is aspecification.

BACKGROUND OF THE INVENTION

The invention relates generally to store price display systems, andrelates more particularly to low-power wireless two-way communicationarrangements in such systems.

An electronic shelf label (ESL) system comprises many individual,addressable ESLs in a store, typically 15,000 or more. These ESLs aresituated in areas according to the organization of the retail store.Specifically, the ESLs are along a shelf edge (in some systems mountedon a rail), and several shelves are associated vertically in a verticalbay. Several vertical bays may be logically associated as a section orcategory, and several sections may be positioned in a half-aisle.Rail-based systems can deliver power to the ESLs, allowing extensivecommunications to the ESLs limited only by the communication bandwidth.However, rail-based systems are best suited to installation on shelvingunits, and present installation difficulties and additional costs wheninstalled in other areas of the retail environment such as peghookdisplays, produce areas, free-standing display tables, deli and meatdisplay cases, wire bins (such as for bakery products), and othergeneral merchandise areas. For these areas, an ESL system using selfpowered ESLs (battery, solar, or other technologies) is highly desiredbecause it offers installation advantages in that the ESLs may bedirectly attached to a variety of retail display fixtures without theimpediment of wiring and the associated installation and maintenancecosts.

Prior art (Pat. No. 5,241,467, Failing et. al., and Pat. No. 5,245,534,Waterhouse et. al., each assigned to the same assignee as the assigneeof the present application, and each of which is incorporated herein byreference) describes means and methods to collect, maintain, and uselocation information on each ESL and the product it represents. Thisinformation is then used to cause all ESLs in an area or sub-area tochange their displays in response to a user request initiated by a handheld unit, a special purpose module, an initiator, or a display functionswitch. In the current art, multiple messages must be prepared and sent,one each to each ESL in the desired area, to effect the desired displaychange. In a dense area, such as Health And Beauty Aids (HABA), the timenecessary to address all the ESLs in a section or several adjacentvertical bays may take several tens of seconds, too long to allow forefficient in-aisle inter-activity with ESL-displayed store maintenanceinformation, such as Computer-Aided Ordering (CAO), shelf or spacemanagement, inventory management, or promotional or merchandisinginformation. In addition, for a power limited system, such as an RF orIR system powered by solar cells or batteries, it is desirable tominimize the number of transmissions from each ESL and, moreimportantly, to minimize the receiver-on time (during which the labelwaits for a possible message), in order to conserve power and extendbattery life.

Co-pending patent application Ser. No. 08/201,470 (assigned to the sameassignee as the present application and incorporated herein byreference) for Automatic Merchandising Audit Systems is directed toadditional applications for shelf edge labels that also requireincreased communications to the labels. These applications requirereasonably rapid response by the system to the operational actions beingtaken by the store employee in the aisle. In order to avoid long delays,and the resulting increased labor costs waiting for the label response,the receivers must all be turned on, or at least the label receivers inthe geographic section in which the activity is occurring must be turnedon, again consuming the energy from the battery. Co-pending applicationSer. No. 08/247,334, Sub-Global Area Addressing (assigned to the sameassignee as the assignee of the present application, and incorporatedherein by reference), attempts to address this issue by providing meansby which groups of labels may be quickly activated while minimizing thetotal number of labels required to have receivers on. This solutionprovides some benefit in trading off response time of a limitedgeographic area with battery life, it requires some additional softwareoverhead to implement.

The primary limitation of a typical fully functional wireless electronicshelf label is the receiver on time. Transmissions in response toqueries are typically short, perhaps tens of milliseconds, so that thetotal energy used in a transmission is relatively small. However, thereceiver must be on for relatively long periods on order to be ready foran unexpected message, such as a price change or another display changeto support in-aisle activities such as merchandising, computer aidedordering, or space management. Co-pending patent application Ser. No.08/258,409, Low-Powered, RF-Linked Price Display System (assigned to thesame assignee as the assignee of the present application, andincorporated herein by reference), addresses the power limitation bycombining passive RF transceiver technology with a battery powered (oralternatively solar powered) electronic shelf label with a liquidcrystal display (LCD). The technology is well known to operate suchdisplays for the required 5 to 7, perhaps longer, years using aneconomical lithium coin cell. By implementing a passive RF transceiverwith the ability to receive a message when energized, and alert the LCDcontroller through an interrupt line when receiving a message, themessage can be captured by the electronic shelf label without expendingany more than a minimum amount of battery energy when transferring thereceived data to the label. In an alternate configuration, the passiveRF transceiver need not store any of the data except its own unique ID(or sub-global IDs if used), but needs only to wake up the electronicshelf label when a message is to be received.

The use of passive RF transceivers or RF diode detectors biased atacceptably low power levels requires the system transceiverscommunicating with the ESLs to operate at reasonably high power levels.For operation within the United States in the unlicensed ISM(instrument, scientific, and medical) bands, the radiated power islimited to less than 1 milliWatt (FCC 15.249) for unrestrictedmodulation implementations. Within the modulation restrictions in thesebands, unlicensed radiation power levels are limited to 1 Watt (FCC15.247). For a passive ESL transceiver, utilizing the well-known art ofreflective antenna techniques for low power communications to the systemtransceiver, the reliability of the link is greatly dependent on theradiated power level of the system transceiver, since the energy"transmitted" by the ESL is the reflected energy received at the ESLantenna from the system transceiver. This is due to the level ofreceived signal from the ESL at the system transceiver falling off atleast as quickly as the fourth power of the range between the systemtransceiver and the ESL.

The radiated power level of the system transmitters can be increased,since these units are likely communicating directly with the host (bywire) and are therefore easily powered from a source connected to the ACpower mains and not subjected to battery limitations. However, withinthe United States radiation power levels above 1 milliWatt are subjectto modulation and operation restrictions, and radiation power levelsabove 1 Watt are not authorized. A solution is to obtain site licensesfrom the FCC for operation at higher power levels using the desiredmodulation techniques. (Not all modulation techniques are equallyapplicable due to the low cost and low power requirements of the ESL).Operational licenses for radiated power levels up to 8 Watts have beenauthorized (on an experimental basis) for this purpose. However, sitelicensing is not a very practical solution, since the licenses must berenewed on a regular basis (typically 5 years) on a site-by-site basis.This could present an annoying paperwork burden on a supermarket retailchain of several hundred to well over 1000 stores. Additionally,increasing health concerns by the medical authorities, government(OSHA), and consumers about the safety of microwave radiation make thedecision to increase power levels arbitrarily an undesirable one. Thisis made worse by the fact that the desire would be to select a physicallocation for the system transceivers that is as low as practical inorder to reduce the effects of the path loss (fourth order of range),thus placing the transmitters even closer to the consumers and storeemployees. An alternate solution might be to operate at lower, thereforesafer, power levels, but this would require that additional systemtransceivers be installed in the ceiling, thus increasing theacquisition, installation, and maintenance costs of the system.

Modulation techniques used by the current technology of passivecommunication systems using the reflective impedance of antennas haveadditional limitations in the system transceiver. Typically, thesesystems use a double-sideband amplitude modulated (DSB AM) signal forthe downlink from system transceiver (transmitter) to the ESLtransceiver (receiver or detector). For uplink communications, thesystem transceiver continues to transmit a continuous wave unmodulatedcarrier signal and in the ESL, an RF device modulates the impedance ofthe ESL antenna with a local oscillator. Oscillator frequencies of 10 to20 kHz are typical. The data are transmitted by applying double-sidebandAM modulation to the local oscillator, thus creating modulated sidebandson either side of the carrier signal separated by the local oscillatorfrequency. Typical local oscillator frequencies would be about 16 kHz(easily obtained from low-cost 32.768 kHz watch crystals).

The system is further complicated by the fact that the receivedreflective modulation signal will typically have a signal level of lessthan -30 dBm, while the transmitted unmodulated carrier, which has to beoperating simultaneously in order to produce the reflective modulation,will typically be operating at a power level of about +30 dBm. Since thepath loss for this transmission falls off at the fourth power of range,combined with the desire to minimize the total system costs (acquisitionand installation), the system transmitter and the system receivingantenna must be located in close proximity. This means that the receivermust reject a carrier signal 60 dB higher than the maximum expectedreceive signal, and the carrier signal distance in frequency from thecenter of the receiver bandwidth is of the order of the bandwidthitself. This requires extremely steep skirts on the receiver bandpassfilter, or an accurate notch filter, and precludes relaxation of thecarrier frequency tolerance for cost reduction purposes. Accommodationis limited, because moving the receive antenna closer to the transmitterincreases the interference from the carrier, and increasing theseparation reduces the received reflective signal.

What is desired is a transmission scheme that radiates at lower powerlevels to be safe for consumers and store employees, does not requiresite licensing, has reasonable coverage to minimize the number of systemtransceivers needed to cover an area of interest (the store), and stillprovides reliable communications at low cost and low power levels with abattery powered ESL (electronic shelf label).

SUMMARY OF THE INVENTION

The invention consists of a system the elements of which include one ormore (typically ceiling-mounted) transceivers and a multiplicity of ESL(or tag) receivers/retroreflectors. The system transceiver transmittedsignal is a double sideband amplitude modulated (ON-OFF Keyed)transmission with an underlying Direct Sequence Spread Spectrum (DSSS)Pseudo-Random sequence of N chips (N>10) for each data bit thatmodulates the carrier (preferably around 2.4 Ghz) in Binary Phase ShiftKeying (BPSK). Another way to describe the chipping rate is that it isdesirably at least ten times that of the data rate. The ON-OFF keyedmodulation for the information to be transmitted to the ESLs preservesthe average duty cycle of the carrier regardless of the informationtransmitted through Manchester encoding. The transmitter operates inaccordance with FCC Parts 15.247, 15.205, and 15.209, keeping theradiated power level below 1 Watt and meeting the requirements forunlicensed operation.

The invention employs a low-cost ESL "receiver", consisting of a simpleantenna tuned to the center frequency (the above-mentioned 2.4 GHz forexample), a low-cost Schottky or PIN diode, a crystal oscillator using a1 to 10 MHz crystal (with a tolerance of ±100 parts per million), andsome passive components. The front end contains an antenna and diode.The diode functions as a detector for RF reception, and (as will bedescribed below) functions also as a mixer and modulator for uplinkresponse. When the ESL receives, the RF energy collected by the tunedantenna is detected by the slightly biased diode. As mentionedpreviously, the downlink data modulation is ON-OFF keying. The diode, inconjunction with the demodulator (which is implemented in the low-powerlogic) extracts the transitions representing the incoming message andfeeds the result into a receive shift register. The demodulator consistsof an amplifier and a Manchester decoder.

When the ESL "transmits", the diode is used as a device that mixes the(spread spectrum direct sequence modulated) signal received from thesystem transceiver (which is not modulated with any data) with theinternal (10.7 MHz for example) oscillator signal, thereby generatingtwo frequency components, one above and one below the received signal,each shifted by 10.7 MHz. Transmitted data is modulated by inverting thephase of the 10.7 MHz oscillator, thereby DBPSK (differential BinaryPhase-Shift Keyed) modulating the reflective energy. The 10.7 MHzoscillator operates only during the transmission, conserving the energysource of the ESL. Stated differently, the 10.7 MHz oscillator ispowered off when no label transmission is taking place.

The system transceiver receiver coherently correlates the receivedreflective signal with the same pseudo-random sequence used in thetransmission from the (typically ceiling-mounted) transmit antenna. Thereceived signal from the ESL (or tag) consists of the signal transmittedfrom the system transceiver multiplied by the 10.7 MHz DBFPK modulateddata stream from the ESL. The signal from the (again typicallyceiling-mounted) receive antenna is amplified in a low-noise amplifier,down-converted to an intermediate frequency (IF) by being mixed with thesignal at the transmit antenna, and band-pass filtered. The mixingresults in a correlation with the same pseudo-random sequence used tomodulate the carrier. The correlated signal is converted to baseband,sampled and quantized, and processed using readily available DigitalSignal Processor integrated circuits. DSP algorithms are used to extractbit and frame timing, carrier phase and frequency errors, and the ESLmodulating information. Because the received signal is shifted by 10.7MHz, the receive antenna may be located approximately closer to thetransmitter than it would if the shift were by a smaller offset.Increasing the oscillator frequency allows even closer configurations,since the rejection of the transmitted signal becomes easier and lessexpensive. By carefully selecting the system parameters (ESL subcarrier, spreading sequence length, chip rate, and data rate), thesystem may be optimized for antenna design, antenna separation, andsignal recovery. Proper selection of these parameters can significantlyease the restriction on installation configurations that would otherwiseprevail.

DESCRIPTION OF THE DRAWING

The invention will be described with respect to a drawing in severalfigures, of which:

FIG. 1 shows in block diagram form the label transmit data path in oneembodiment of the invention;

FIG. 2 shows in block diagram form the label receive data path;

FIG. 3 shows an alternative embodiment for the label in FIG. 2;

FIG. 4 shows in block diagram form the circuitry at one pair of receiveand transmit antennas in the store ceiling;

FIG. 4A shows in block diagram form an alternative embodiment for thecircuitry at one pair of receive and transmit antennas in the storeceiling;

FIG. 5 shows in block diagram form part of the circuitry of oneelectronic shelf label;

FIG. 6 shows in block diagram form all the circuitry of one electronicshelf label;

FIG. 7 shows in block diagram form the multiplexer/demultiplexercircuitry for the ceiling-mounted antennas;

FIG. 8 shows in block diagram form exemplary components of the storeelectronic shelf label system;

FIG. 9 shows a time line for important signals in the communications;

FIG. 10 shows a perspective view of a label according to the invention;

FIG. 11 shows a cross section of the label of FIG. 10;

FIG. 12 shows in perspective view the main printed circuit board for thelabel of FIG. 10;

FIG. 13 shows a multipath model; and

FIG. 14 shows a second multipath model.

Where possible, like elements have been denoted with like referencenumerals.

DETAILED DESCRIPTION

The system according to the invention may perhaps be most clearlyintroduced by initial reference to FIG. 8. A multiplicity of electronicshelf labels 95, 95' etc. are dispersed throughout the store. Each ESLcontains a liquid-crystal display, a processor, a battery, an antenna,and analog circuitry relating to sending and receiving information viathe antenna. The terms "electronic shelf label" and "electronic pricedisplay" are used interchangeably herein.

The ESLs are controlled by a store central computer 93. The computercontains records indicative of the information (e.g. price) that is tobe displayed by each label. The store is served by several pairs ofantennas 13, 22 which effectively divide the store into "cells". Eachcell is served by one pair of antennas 13, 22, each such pair iscontrolled by a controller 91, the controller 91 is connected by a cable62 to a cable interface 63, and the several cable interfaces 63 (one foreach cell) are connected via lines 94 to a multicell controller 90. Themulticell controller is in turn communicatively coupled with the centralcomputer 93 by a bidirectional link 99.

The controllers 91 are shown in more detail in FIG. 4. The multicellcontroller 90 is shown in more detail in FIG. 7. The label 95 is shownin more detail in FIG. 6, and the interface chip 52 of FIG. 6 is shownin more detail in FIG. 5.

Turning now to FIG. 1, there is shown in block diagram form the labeltransmit data path in one embodiment of the invention. The goal for thisdata path is to communicate a data stream on line 19 (within the label95) to the store central computer on received data line 38. This goal,as mentioned above, is to be accomplished consistent with minimizingpower consumption in the label 95.

When it is desired for the label 95 to be able to communicate data, thetransmit antenna 13 is energized. To this end, a carrier oscillator 10is switched on, preferably at a frequency in the neighborhood of 2.4Ghz. A sequence generator 12 is also switched on, generating a binarysequence used for direct sequence spread spectrum modulation. Typicallythe sequence would be N bits in length N>11, chosen such that it has"good" autocorrelation function. The clock rate or "chipping rate" forthe sequence is chosen to be N times the data rate on line 19. Thesequence signal on line 27 is used to control a binary phase-shift-keyedmodulator mixer 11, which in simplified form may be thought of as aswitch that selects either the signal on line 26 or the 180°phase-inverted form of the signal on line 26. The result is a well-knowndirect sequence spread spectrum (DSSS) RF signal. This signal isamplified by power amplifier 14 and the amplified signal on line 28 isbroadcast into an area of the store by antenna 13.

Dotted line 30 denotes the RF energy coupled to antenna 15 through theair between the store transmit antenna 13 and the antenna 15 within thelabel 95. Generally this coupling would be line-of-sight propagationalthough reflection paths may sometimes yield the strongest coupling.

The label processor 21 turns on oscillator 18 via control line 20.Oscillator 18 provides an output on line 29, typically at 10.7 Mhz orpossibly as low as 1 Mhz as mentioned above. An XOR gate 17 uses thetransmit data line 19 to control the phase of the oscillator signal ofline 29 to the diode 16 (preferably a Schottky or PIN diode). Working asan RF switch, the RF diode returns the incoming signal (or rather afraction of it) to the antenna 15 in either one phase or the oppositephase. This switching occurs at a frequency equal to the ESL localoscillator 18, creating the two frequency components around thefrequency of the incoming signal, removed from it by the switchingfrequency. The phase of these two spectral lines is Binary Phase ShiftKeying (BPSK) modulated by the data.

The returned energy propagates along path 31 to receive antenna 22. Whenthe data line 19 is inactive, the energy at antenna 22 carried alongpath 31 is nearly identical to that emitted at antenna 13, and ispresumably dwarfed by the energy propagating directly from antenna 13 toantenna 22. When the data line 19 is active, however, then it isexpected that the energy propagated along path 31 will be detectable dueto its frequency offset.

The energy received at antenna 22 passes through a low-noise amplifier23 and is then mixed in mixer 24 with the same signal (carried on line25) that is being transmitted at antenna 13. This mixing serves twopurposes. First, it acts as a conventional superheterodyne, yielding anintermediate frequency (IF) signal at 34 which is then processed bycircuitry downstream. The superhet reception means that the downstreamcircuitry need not function at gigahertz levels, but instead need onlyfunction at the offset frequency which is only ten MHz or so. Second, itperforms a spread-spectrum despreading, in the sense that the output 34is a conventional-bandwidth signal rather than a spread-spectrum signal.

The IF signal on line 34 is passed through an IF bandpass filtercentered at the offset frequency of the oscillator 18. The filter neednot be too narrow, and desirably is not too narrow, so as to accommodatethe data bandwidth and the frequency variations in the oscillators 18 ofthe various shelf labels. One important effect of the mixing and IFbandpassing is that virtually none of the energy coupled directly fromantenna 13 to antenna 22 reaches line 36. This overcomes one of thetraditional problems that arises when a transmitting antenna 13 isnearby to a receiving antenna 22, namely desensitization of the receiverat antenna 22.

The signal at line 36 is demodulated, and the resulting binary serialsignal 38 is available to the store central computer.

It will be appreciated that the label 95 does not require much power todo all of this. Its oscillator 18 is powered only during transmitintervals and is unpowered at other times. Antenna 15 is merelyreturning energy from antenna 13 in either one phase or the oppositephase. What's more, none of the components in the label (save for thediode 16 and antenna 15) have to be capable of doing anything in thegigahertz range. The list of label components that have to function inthe megahertz range is also quite short; it is the oscillator 18 and thegate 17.

Many classical spread-spectrum receivers have to go through asynchronization exercise to be able to receive a spread-spectrum signal.In the system shown in FIG. 1, however, the phase of the sequence fromthe generator 12 is known to the receiver (via line 25) so no spreadingsequence synchronization exercise or circuitry is required. The directsequence correlation is automatic and the DSSS demodulation isaccomplished quite simply in the mixer 24.

It might be asked why one bothers to transmit a spread-spectrum signalat antenna 13, since theoretically the serial communication from line 19to line 38 works just as well with a conventional-bandwidth(non-spread-spectrum) signal transmitted at antenna 13. If spreadspectrum were eliminated, this would save having to provide the sequencegenerator 12 and the modulator 11, for example. But in a realistic storesetting the RF power level that would be required to make the system ofFIG. 1 work might be too high to satisfy regulatory requirements.Transmitting a signal that has been spread-spectrum modulated takes noteof regulatory provisions that favor spread-spectrum use of the portionbeing utilized.

It will be appreciated by those skilled in the art that numerousspread-spectrum techniques are known which would, in the arrangementshown here, offer the benefits described. These techniques includefrequency hopping and other phase-shifted modulations. But the directsequency binary phase shift keyed modulation is thought to be simpler toimplement.

FIG. 2 shows in block diagram form the label receive data path. Theoverall goal is to communicate a data stream on line 42 (from the storecentral computer) to line 46 (to be made available to the labelprocessor 21). The spread-spectrum RF signal is provided as an input toswitch 43. This is a 100% AM modulated signal, sometimes called anOf-Off keyed (OOK) signal. The signal is transmitted from antenna 13 andreceived at antenna 15 through coupling path 30. The detector diode 16extracts RF energy from the antenna 15, and operates as a simpleenvelope detector. The detected signal is amplified, passed through athreshold device and Manchester decoded at 45 and made available to theprocessor 21 of the label 95.

FIG. 3 shows a more detailed depiction of the signal flow path of thelabel 95. The signal from the diode 16 and antenna 15 is capacitivelycoupled (to permit the amplifiers to operate from a single power supply)to an amplifier which is in turn connected with a comparator. The biasfor the amplifier, and the threshold for the comparator, are bothselected to be halfway between the power and ground levels in the label.The output, which has been implicitly envelope detected, is thenManchester decoded at 45. The use of capacitive coupling is what promptsManchester encoding and decoding since there is no net bias toward alogic 1 or 0 in the data stream.

It will be appreciated that the arrangements of FIGS. 2 and 3 offerseveral benefits. First, they minimize the cost and complexity of thelabels 95. The only circuit elements operating at the gigahertz levelare the antenna 15 and diode 16. Everything downstream-of the diode 16is essentially audio-frequency analog signal or digital binary signal.

To obtain these benefits one pays a price, of course. The first is thatthe "receiver", such as it is, of the label 95, is basically simplymeasuring in a relatively crude way the total RF energy impinging uponthe antenna 15, and the acceptance bandwidth of the antenna is limitedonly by the antenna itself. For this reason the store layout is selectedso that of the many antennas 13 in the ceiling, one is particularlyclose to the label 15. What's more, the multicell controller 90 is setup so that at any particular instant if an antenna 13 is transmitting,the antennas nearby to it (and perhaps all the other antennas 13 in thestore) are silent. The threshold of the comparator within the Manchesterdecoder is set relatively high, so that the ambient store RF level doesnot register.

Here, too, one might ask why the trouble and expense of the spreadspectrum modulation need to be incurred. If the SS modulation could bedispensed with, the generator 12 and modulator 11 could be eliminated,for example. Again, the design takes note of regulatory provisions thatfavor spread-spectrum modulation, since this promotes bandwidth reusewithin a store and nearby to the store.

FIG. 4 shows in block diagram form the circuitry at one pair of receiveand transmit antennas in the store ceiling. Carrier oscillator 10 may beseen, controlled by on/off control line 49. In this embodiment theon-off keying of the serial data signal takes placed in switch 43. (Theserial data signal is received by the controller over line 62.) Next theRF signal, which has already had the serial data imposed upon it, ismodulated with reference to the direct sequence signal from thegenerator 12. Binary Phase-Shift Keying modulator (or mixer) 11 spreadsthe energy of the RF signal over the spread-spectrum bandwidth. Thespreaded signal is amplified and transmitted by antenna 13.

Still with reference to FIG. 4, the spread-spectrum signal is madeavailable via line 25 for data reception. The received RF energy atantenna 22 is amplified and then mixed in mixer 24 as discussed inconnection with FIG. 1. An IF bandpass filter 35 strips off the energyabove and below the IF frequency. This signal is received at cableinterface 60 and carried on cable 62. The controller 91 is one of manycontrollers 91 placed in the ceiling of the store, each with itstransmit and receive antennas 13, 22.

FIG. 4A shows an alternative embodiment for the ceiling cell sitecontaining a transmit antenna 13 and a receive antenna 22. The carrieroscillator output is passed directly from isolator 48 to the BPSK mixer11. The phase-control input to the mixer is the logical AND of the DSSSsequence and the on-off-keyed data stream.

Turning now to FIG. 6, what is shown in block diagram form is thecircuitry of one electronic shelf label 95. A microcontroller chip 21,preferably such as the one made by Sanyo, controls the LCD display 120and stores information to be displayed thereon. The offset oscillator 18is also shown, controlled by line 20. Antenna 15 and nonlinear device 16are also shown. Decoder chip 52 may also be seen. Omitted for clarity inFIG. 6 are a lithium battery, an optional customer pushbutton, andoptional EEPROM memory. Various data and control lines also link thecontroller 21 and the decoder chip 52. As mentioned above the nonlinearelement 16 is preferably a Schottky or PIN diode but could optionally beany of a wide range of two-terminal nonlinear devices capable ofoperation at microwave frequencies.

Turning now to FIG. 5, what is shown is the internal block diagram forthe decoder chip 52. RF energy received at the antenna is envelopedetected by the RF diode 16 (of FIG. 6) and communicated via line 50 tofront end amplifier 44. Because the signal has been Manchester encoded,the signal may be capacitively coupled. Manchester decoder 54 is seen.Received data, depending on its content, is made available to theprocessor via data lines 59. The label transmit data path is also viabidrectional data lines 59, and tranmission is commenced by control line121, which starts the transmit state machine 122. Under control of thestate machine 122, the data to be sent are clocked out of shift register123. A CRC generator 399 generates a CRC which is appended to the data,a step also controlled by the state machine 122.

FIG. 7 shows in block diagram form the multiplexer/demultiplexer andmulticell controller circuitry for the ceiling-mounted antennas. Each ofseveral cable interfaces 63 is connected to particular ports inmultiplexer 65 and demultiplexer 64. Data bound from the centralcomputer (omitted for clarity in FIG. 7) to the labels is sent via line75 to one of the cell controllers. It is translated to an intermediatefrequency of 455 kHz in order to enable the utilization of the cable 62for a multiplicity of data paths. The received IF signal from a selectedone of the cells is converted down to complex baseband signal afterbeing filtered by IF bandpass filter 66, amplified and split into twocomponents with amplifier 67 and a power splitter. Two mixers, 69 and 70convert the IF signals into two quadrature components whereby they aresampled and quantized by the A/D (analog-to-digital) convertors 72 and73. In this way both the amplitude and the phase of the IF signal arepreserved in the sampled data.

The DSP (digital signal processor) uses these samples to syncronize anddemodulate the received signal using digital signal processingalgorithms that implement the operation of a packet modem.

The serial data from one of the labels 95 is then made available to thecentral computer.

FIG. 9 shows a time line for some important signals. Line 13 shows theRF power present at antenna 13. Bursts of power 101 on that line aredetected within each label 95 and processed as serial digital signals 46in each label. Most of the labels ignore the signal, as set by protocolsdiscussed in applications incorporated herein by reference, namelyNon-slidable Display Label, application Ser. No. 07/965,877, filed Oct.23, 1992; Technique for communicating with electronic labels in anelectronic price display system, application Ser. No. 07/995,048, filedDec. 22, 1992; Electronic price display system with data bus isolation,application Ser. No. 08/008,200, filed Jan. 25, 1993; Technique forlocating labels in an electronic price display system, application Ser.No. 08/031,580, filed Mar. 15, 1993; Information Display Rail System,application Ser. No. 08/036,950 filed Mar. 25, 1993; Electronic pricedisplay system with vertical rail, application Ser. No. 08/045,910,filed Apr. 12, 1993 issued as U.S. Pat. No. 5,348,485 on Sep. 20, 1994;Space Management System, application Ser. No. 08/114,510, filed Aug. 31,1993; Electronic Shelf Label Location System, application Ser. No.08/155,723, filed Nov. 22, 1993; Shelf Talker Management System,application Ser. No. 08/201,470, filed Feb. 24, 1994; Technique forlocating electronic labels in an electronic price display system,application Ser. No. 08/207,956, filed Mar. 8, 1994; System for LocatingDisplay Devices, application Ser. No. 08/210,046, filed Mar. 17, 1994;Display System with Section Addressability, application Ser. No.08/210,163, filed Mar. 17, 1994; and Subglobal Area Addressing forElectronic Price Displays, application Ser. No. 08/247,334, filed May23, 1994, all assigned to the same assignee as the assignee of thepresent application.

When the store central computer wishes to receive data from a label, itsends a command to that label instructing the label to respond. Thelabel 95 prepares its response. The central computer then causes anon-data-modulated carrier 103 to be broadcast for an interval of timesufficient to permit the response from the label. During that interval200, the label 95 turns on its oscillator 18, and sends the serial datastream 106 on line 19. This data stream DBPSK modulates the oscillatoroutput at 29 and this modulated signal is applied to the diode, causingthe reflected RF signal 104 to be different (offset) during the interval200. During the time that both 103 and 200 exist, an offset signal isreceived at 22 and arrives at line 36 with the data modulating its phase107. This signal is demodulated by 37 and the serial data 108 areproduced at line 38.

An antenna design for the label is shown in FIGS. 10, 11, and 12. FIG.10 shows an electronic price display label with plastic case 126. Thelabel may be mounted next to a relatively unpredictable variety of metaland plastic store shelves, and may be attached directly to a metal orplastic rail on the shelf edge. FIG. 11 shows a cross section of thelabel. The antenna, shown in exaggerated scale for clarity in FIG. 11,is at 15, with the diode 16 nearby. A slot antenna ground plane 124,also not shown to scale, appears on the other side of printed circuitboard 122. One side of the PC board holds the digital signal extractionchip 52 (see FIG. 6) and the other side holds the microprocessor chip 21(see FIG. 6). The liquid crystal display 120 is mounted at the front ofthe label. FIG. 12 shows in perspective view the portion of the PC board122 containing the antenna 15. A slot ground plane 124 is on the lowerface of the PC board. A feedthrough 127, preferably a platedfeed-through, provides a connection from the ground plane 124 to the topside of the board 122, for connection with the diode 16. The groundreference is also made available to the signal extraction chip 52. Theother end of the diode 16 is connected with a tuned stub antenna 15 andwith an antenna feed line to the signal extraction chip 52. The physicalstructure just described comprises a slot antenna. The dimensions areselected for impedence and frequency matching. In an exemplaryembodiment the ground plane is about 1 inch wide and 1.375 inches tall,with a 0.25 inch slot etched out in the center of the rectangle. Theslot does not go all the way to the bottom of the rectangle but isnominally one-quarter of the wavelength of the carrier signal. The diode16 is nominally about one-quarter inch up from the bottom of the slot.The position is sensitive and is empirically set with millimeterprecision for impedence matching. The anode is connected to the antennaelement, which is about one inch long. A reflector 397 made of thinmetal is optionally placed at the rear of the label as shown in FIG. 11to swamp out any variable effects from the rear such as metal shelvingand the like. The reflector also allows fine-tuning the antenna and slotplane for desired propagation patterns that are repeatable even withvarying mounting conditions.

In an alternative embodiment of the invention, a provision is made toovercome multipath phenomena. Turning to FIG. 13, one model may be seenwhereby a multipath problem could arise. The transmitting antenna 13,which is typically in the store ceiling, emits over a wide area fromwhat is more or less a point source. Line 150 shows a straight-linepropagation path from the antenna 13 to a typical label 95. Line 151shows a possible second path of propagation from the antenna 13 to thelabel 95. If the difference in the two path lengths happens to be at ornear a half-integral multiple of the transmit frequency, then there isthe possibility of destructive interference. Since a typical frequencyis 2.4 Ghz, the wavelength is about 0.13 meters or about five inches.Experience shows that the dimensions and surfaces present in a retailstore can indeed provide nodes (areas where destructive interferenceoccurs).

FIG. 14 shows a second model for multipath interference. The signalreflected from a label 95 to the ceiling receiver 15 is, in the simplecase, a straight-line signal 152. It is to be expected, however, thatreflected paths 153 may present themselves, and that in some cases thedifference in the path lengths may again be at or near a half-integralmultiple of the return signal frequency. (It will be appreciated thatthe reflected energy is offset in frequency relative to the downlinkenergy so the relevant wavelength and/or wavelengths for multipathphenomena in the uplink are different than for the downlink.)

It will be noted that in a retail store, considering the wavelengthsthat are likely to be used, multipath interference is due to a mix offixed and changing conditions. Fixed conditions include the locations ofshelves, floor, and ceiling. Changing conditions include locations ofmerchandise, shopping cards, free-standing displays, and the like.

In many prior-art systems of this general type, the usual approach tomultipath problems is to decrease the cell size and thus increase thearea density of ceiling antennas, so that each label 95 is reachable bymore than one ceiling antenna. When a label is unresponsive (which maybe due to multipath or other causes), the system then tries a differentcell antenna. The assumption is that the physical objects giving rise tothe multipath effect will be unlikely to give rise to a multipath nodefor the different antenna. A second approach is to establish severaldifferent frequencies to be used for the downlink and/or uplink. Eachfrequency will, of course, have a different wavelength and thus willhopefully not suffer the same multipath loss as some other frequencymight suffer. This approach, in many systems, is awkward because itrequires coordination of frequencies in the ceiling site and in thelabel. The label must have hardware provisions for selection of uplinkand downlink frequencies, as must the ceiling site. Some class of device(the ceiling site or the label) has to choose the frequency of themoment, and the other of the two devices has to be told what frequencyis to be used, or has to stumble upon it by polling or some sort ofsynchronization schedule. This consumes some of the uplink and/ordownlink bandwidth.

According to the invention, however, the multipath problem is readilyremedied by the step of changing the transmit frequency generated by thetransmit oscillator 10 (FIG. 4). Control line 49 includes informationindicative of the desired transmit frequency, controlled by the storecentral computer or by distributed intelligence in the store. Returningto FIG. 1, it is noted that antennas 13, 15, and 22 are all fairlywideband; stated differently, there is nothing about the designrequirements for the antennas that would call for any of the antennas tobe narrowband. Furthermore, nothing about the circuitry of label 95 isparticularly tied to the precise frequency from the oscillator 10. Inother words, nothing about the label 95 has to change to accommodate thechange in the frequency from the oscillator 10. Finally, since theenergy received at ceiling antenna 22 is mixed with the energy of line25 (the same signal that is transmitted at antenna 13), then nothingabout the receive signal path 22, 32, 23, 24, 35 has to change toaccommodate the change in the frequency of the transmit oscillator 10.

It will thus be appreciated that remedying multipath problems is mucheasier in the system of the invention, as compared with other prior artsystems.

Consider, then, what happens if the store computer 93 (FIG. 8) wants tocommunicate a message to a label 95. The message is sent via thedownlink path, shown as 30 in FIG. 1 or as 150/151 in FIG. 13. The labelis then asked to respond, and the response is propagated via the uplinkpath, shown as 31 in FIG. 1 or as 152/153 in FIG. 14. Experiencesuggests that multipath will be encountered in only about 15% or less ofinstances. If there is no response, this may be due to uplink multipath,downlink multipath, or due to something else. The store computer thencommands the oscillator 10 to shift to a new frequency, and the two-waycommunication is again attempted. Experience suggests that in only about15% or less of instances will this link also fail due to multipath. Thusin two tries there is only about a 2% likelihood of multipathinterfering with both tries. A shift to a third frequency fromoscillator 10 is quite likely to overcome that residue of unsuccessfulcommunication attempts.

It is important to note the benefits of the system according to theinvention. For example, the shifts in frequency of the oscillator 10need not be particularly closely fixed. That is, the shift could besubstantially more or less than some nominal shift, and communciationwould still take place. As mentioned above, this is due to (1) therelatively wideband nature of the signal paths; (2) the absence of anyparticular frequency dependence or tuning in the label 95; and (3) thefact that the receive path uses mixing at 24 based on the transmittedsignal of line 25, thus canceling out any drift or lack of repeatabilityin the frequency shift of oscillator 10.

If desired, the store central computer 93 (FIG. 8) can keep note, foreach label, of the frequency that worked the last time the label wascommunicated with. Subsequent attempts to reach a particular label canstart using the frequency that worked last time. (It is to be noted thatthere is no assurance a particular frequency will work with a label asecond time, since shopping cards, merchandise, and other items may movein the interim.)

Those skilled in the art will appreciate that what has been described isa frequency-agile system for uplink and downlink communciations betweenceiling-mounted antenna (or plurality of antennas) and one or moreelectronic price display labels, in which the downlink and uplinkfrequencies all shift by some offset amount, and yet no hardware changeis required in the label or in the ceiling receive signal path toaccommodate the shift, and the only hardware variation required is ashift in the output frequency of the ceiling transmit oscillator 10. Inparticular nothing about the label has to explicitly provide for suchshifts, nor does any intelligence in the label need to be aware thatsuch shifts have ocurred; this furthers the general goal ofcost-reducing the labels to the greatest extent possible. Those skilledin the art will appreciate further that this represents an exceedinglyinexpensive and hardware-efficient approach to remedying multipathinterference problems, especially when compared with prior artapproaches to the problems.

Alternatively the central computer can simply always try a particularfrequency first for all labels, and then shift to a second frequency forany labels that failed to respond to the first frequency, and so on.

Those skilled in the art will appreciate that numerous obviousmodifications and variations could be made to the invention withoutdeparting from it in any way. All these obvious modifications andvariations are intended to be encompassed within the scope of theclaims.

What is claimed is:
 1. A communications system for communicating aserial data stream from a first device to a second device, the first andsecond device being physically separate from each other;the seconddevice comprising a transmitter transmitting an RF signal modulated by aspread-spectrum modulation; the first device comprising an antennacoupled to a two-terminal nonlinear component, an oscillator, theoscillator having an output defining an offset frequency; and a mixerresponsive to the serial data stream for selectively coupling theoscillator output to the nonlinear component in response thereto; andthe second device further comprising a receiver receiving RF energypresent at a sideband offset from the transmitted signal by the offsetfrequency, and a spread-spectrum demodulator demodulating the receivedRF energy with respect to the spread-spectrum modulation of thetransmitter, and a second demodulator demodulating the demodulatedsignal, yielding the serial data stream.
 2. The system of claim 1wherein the spread-spectrum modulation is direct sequence modulation,wherein the transmitter further comprises a sequence generator with anoutput, the sequence output controlling a first phase inverter whichselectively inverts the phase of the transmitted RF signal.
 3. Thesystem of claim 1 wherein the spread-spectrum modulation isfrequency-hopping modulation.
 4. The system of claim 3 wherein thespread-spectrum demodulator demodulates the received signal by mixingthe transmitted RF signal with the received RF signal.
 5. The system ofclaim 4 further comprising an intermediate-frequency bandpass filterfiltering the output of the mixer.
 6. A method for communicating aserial data stream from a first device at which the serial data streamoriginates to a second device, the first and second device beingphysically separate from each other, the first device compising anantenna and a nonlinear component coupled thereto, the method comprisingthe steps of:generating at the second device a spread-spectrummodulation; transmitting from the second device an RF signal modulatedby the spread-spectrum modulation; receiving energy of thespread-spectrum-modulated RF signal at the antenna of the first device;generating at the first device an offset signal at a frequency definingan offset frequency; selectively coupling at the first device the offsetsignal to the nonlinear component in response to the serial data stream;receiving at the second device RF energy at a sideband offset from thetransmitted signal by the offset frequency; spread-spectrum demodulatingat the second device the received RF energy with respect to thespread-spectrum modulation generated at the second device; and furtherdemodulating the demodulated signal at the second device, yielding atthe second device the serial data stream that originated at the firstdevice.
 7. The method of claim 6 wherein the spread-spectrum modulationis direct sequence modulation, wherein the transmitter further comprisesa sequence generator with an output, the sequence output controlling afirst phase inverter which selectively inverts the phase of thetransmitted RF signal, yielding the RF signal modulated by thespread-spectrum modulation.
 8. The method of claim 6 wherein thespread-spectrum modulation generated at the second device isfrequency-hopping modulation.
 9. The method of claim 8 wherein thespread-spectrum demodulation at the second device is performed by mixingthe transmitted RF signal at the second device with the received RFsignal from the first device.
 10. The method of claim 9 furthercomprising the step, performed after the spread-spectrum demodulationstep, of filtering the output of the mixing through anintermediate-frequency bandpass filter.